The Efficiency of Nonthermal Particle Acceleration in Relativistic Magnetic Reconnection

One of the major questions under investigation in relativistic magnetic reconnection is the heating and acceleration of particles and the energy partition between them. Using fully-kinetic PIC simulations, we evaluate the contributions of three important particle injection mechanisms (parallel electric field, Fermi, and pickup processes) across a parameter scan of varying guide field strength and domain size. Our analysis is based on injection energies, which we systematically derive from a power-law spectrum fitting procedure. From this analysis, we find that for weak guide fields, Fermi reflections account for about half of the injected particles, while pickup acceleration and parallel electric fields are subdominant. For a strong guide field (bg~1), parallel electric fields inject most of the nonthermal particles. In terms of the acceleration efficiency, we find that for weak guide fields, injected particles comprise ~40% of all downstream particles and possess ~90% of the downstream particle energy. For a strong guide field, injected particles comprise ~15% of all downstream particles and possess ~60% of the downstream particle energy. These results highlight the crucial role of guide field strength in controlling the efficiency of particle acceleration from magnetic reconnection. We also show convergence of acceleration efficiency and related key parameters with increasing domain size, which will help explain the nonthermal acceleration and emissions in high-energy astrophysics.